ATP/ADP cycle

Figure 2.8 The ATP/ADP cycle. The major ATP-generating process from fuel oxidation is oxidative phosphorylation driven by electron transport in the mitochondria. In muscle, the major energy-requiring process is physical activity. The phosphate ion is omitted from the figure for the sake of simplicity.

The free energy liberated in the hydrolysis of ATP is harnessed to drive reactions that require an input of free energy, such as muscle contraction. In turn, ATP is formed from ADP and P when fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs. This ATP—ADP cycle is the fundamental mode of energy exchange in biological systems. 

Figure 14.8. ATP-ADP Cycle. This cycle is the fundamental mode of energy exchange in biological systems.

The basic principle of the ATP-ADP cycle is that fuel oxidation generates ATP, and hydrolysis of ATP to ADP provides the energy to perform most of the work required in the cell. ATP has therefore been called the energy currency of our cells. Like the... 

According to the first law of thermodynamics, the energy (cal) in our consumed fuel can never be lost. Consumed fuel is either oxidized to meet the energy demands of the basal metabolic rate + exercise, or it is stored as fat. Thus, an intake of calories in excess of those expended results in weight gain. The simple statement, If you eat too much and don t exercise, you will get fat, is really a summary of the bioenergetics of the ATP-ADP cycle. 

Describe the ATP-ADP cycle of energy exchange in biological systems. 

Incubate the labeled ATP with myosin and measure the amount of labeled inorganic phosphate liberated as a function of time without actin. Then add actin. The result should be a burst in the amount of labeled Pj released. (See Figure 34.18 in the text for the ATP/ADP cycle.)... 

After discussing the utilization of ATP in the cell it is necessary to consider the other part of the ATP/ADP cycle, i.e. the energetics and mechanism of ATP synthesis. ATP synthesis from ADP and phosphate requires the input of a considerable amount of free energy. Sufficient free energy to reverse ATP hydrolysis may be derived from the oxidation of nutrients such as carbohydrates... 

Reversibility of the ATP Synthase— There are two directions in ATP Synthase system and these two directions correspond to two different functionalities and behavior. This two-way behavior is because of the reversible nature of the ATP-ADP cycle and the structure of the ATP Synthase. Let us term the forward direction as when the Fq drives the y subunit (because of proton motive force) of Fj and hence ATP synthesis takes place. And the backward direction is when hydrolysis of ATP counter-rotated the y subunit and hence the Fq motor and leads to pumping back the protons. Therefore, the forward direction... 

Figure 3. Overview of heat production in a simplified metabolic scheme during heterotrophic (organisms using organic substrates both as source of carbon and electron donor, i.e. energy source) growth. For simplicity, energy transformations are only exemplified by ATP turnover, i.e. the ATP/ADP cycle. The flow of carbon from 1 unit of substrate through the catabolic and anabolic (Ja ab) routes is of simplicity presented as two separate routes.

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